Ordering process of Al5Ti3, h-Al2Ti and r-Al2Ti with FCC-based long-period superstructures in rapidly solidified A1-rich TiA1 alloys

Change in microstructure and stability of superstructural phases in Al-rich TiAl alloys containing 58.0-62.5 at.% Al were investigated using melt-spun ribbons. Ordering processes of long-period ordered phases such as Al5Ti3, h-Al2Ti and r-Al2Ti in the Ll(0) matrix during annealing were examined. The presence of Al5Ti3 and h-Al2Ti phases in the Ll(0) matrix was confirmed in melt-spun Ti-60.0 at.% Al and Ti-62.5 at.% Al ribbons by electron diffraction patterns, while diffuse scattering corresponding to the Al5Ti3 superstructure appeared in Ti-58.0 at.% Al ribbon. In Ti-58.0 at.% Al ribbon, the Al5Ti3 phase developed as an island in the Ll(0) matrix having an obscure coherent boundary at and below 800 degrees C, while it dissolved during annealing above 800 degrees C. Although the r-Al2Ti phase was finally formed as an equilibrium phase, the ordering of Al(5)T(i)3 and metastable h-Al2Ti phases in Ti-60.0 at.% Al and Ti-62.5 at.% Al ribbons occurred prior to the precipitation of the r-Al2Ti during annealing below 800 degrees C. The priority for the ordering process is discussed on the basis of crystal symmetry and periodicity of Al layers parallel to the (002) plane. The anti-phase boundaries (APBs) based on the Al5Ti3-type ordering were observed along {110) planes in Ti-62.5 at.% Al ribbon annealed at 700 degrees C and their energies were calculated using the interaction energy between neighbouring atoms. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.